I. FIELD OF THE INVENTION
The present invention is directed to hard and abrasive cutting or drilling tools comprising a relatively thin polycrystalline diamond or cubic boron nitride (CBN) ruble having a substantially thicker cemented carbide substrate support.
More specifically, the present invention is directed to a method of manufacturing such tools which minimizes the depletion of tungsten carbide-cobalt eutectic from the sintered carbide substrate into the diamond or CBN table. Simultaneously a method of inhibiting undesirable diamond or CBN abnormal grain growth in the transition region of diamond or CBN to carbide bonding is incorporated in the high pressure/high temperature (HP/HT) sintering operation.
II. DESCRIPTION OF THE PRIOR ART
There are numerous prior art patents which describe the making of high pressure/high temperature (HP/HT) polycrystalline diamond or CBN tables with cemented tungsten carbide supports or substrates.
U.S. Pat. No. Re. 32,380 describes composite super abrasive tools comprising a polycrystalline diamond layer wherein substantially all of the diamond grains are bonded directly m adjacent grains and a typically cobalt bonded tungsten carbide substrate. This substrate is substantially larger in volume compared to the volume of the diamond layer. This patent also teaches that the cobalt in the cemented carbide substrate is made available in the HP/HT process as both a binder for the tungsten carbide and as a solvent/catalyst necessary for conversion of graphite to diamond. Although tools made according to the teaching of the above patent are suitable for some applications, the uncontrolled infiltration of cobalt from the WC/cobalt substrate into the diamond table leaves excessive cobalt between the diamond particles, which results in less than optimum abrasion resistance of the diamond layer. The physical and mechanical properties of the cemented carbide substrate at the diamond/carbide interface are also seriously reduced due to cobalt depletion from the carbide.
U.S. Pat. No. 4,411,672 describes a method to overcome the above problems by placing a thin metallic material having a melting temperature lower than the eutectic point of the tungsten carbide/cobalt composition between the diamond powder/carbide interface. The assembly is heated to the melting temperature of the metallic material, but lower than the melting temperature of the cobalt/tungsten carbide composition. This allows a somewhat controlled infiltration of the diamond particles with catalyst/solvent material without undue depletion of the cobalt from the carbide substrate. This method can also supply an excessive amount of cobalt to the diamond layer, thereby reducing its abrasion resistance.
Another means of controlling the amount of metallic infiltrant into the diamond layer is described in U.S. Pat. No. 4,440,573 in which a mass of diamond particles and a mass of infiltrant metal are separated by a higher melting point metallic barrier layer, whereby the infiltrant metal is regulated to flow around the barrier metal layer into the diamond particle layer under HP/HT conditions. Although this method does somewhat improve the flow of metal into the diamond table, abnormal diamond grain growth is still a problem. This condition creates a large mismatch of thermal expansion causing very high stresses with subsequent internal cracks in the sintered diamond or cubic boron nitride layer.
U.S. Pat. No. 5,022,894 teaches a method for making diamond and CBN compacts which comprise positioning a catalyst metal disc and a barrier metal disc intermediate a diamond or CBN and a carbide mass. The catalyst metal disc is adjacent to the diamond or CBN layer and the barrier disc is adjacent to and intermediate the catalyst disc and carbide mass under HP/HT conditions. This method provides regulated flow of metal bond from the carbide mass and helps to minimize depletion of metal bond in the carbide near the catalyst/diamond interface, but surplus catalyst metal in the diamond table can still be detrimental.
U.S. Pat. No. 4,474,434 teaches a method of regulating the flow of cobalt from the tungsten carbide substrate to the diamond or other super-hard material table, with the substrate bonding face having a non-planar or complex geometry. A thin (greater than three microns) continuous layer of refractory material (titanium nitride) is applied by PVD or CVD proximate the diamond/carbide interface. This thin coating of refractory material acts as a fairly effective cobalt/carbide eutectic diffusion barrier and reduces the depletion of cobalt in the carbide substrate. Relatively good control of the binder phase (Co, Ni, Fe) in the diamond/CBN layer is accomplished as one of these binder metal powders is added to the diamond powders prior to sintering, but abnormal tungsten and diamond crystal growth at the diamond/substrate interface remains a problem.
U.S. Pat. No. 4,403,015 teaches the making of a diamond or cubic boron nitride (CBN) composite compact by using an intermediate bonding powder layer between a carbide substrate and a hard diamond or CBN cutting layer. This intermediate layer being comprised of CBN powder admixed with powders of the borides, nitrides, carbides or carbonitrides of the 4a, 5a, 6a transition metals of the periodic table, or an admixture thereof. Diamond or CBN hard layers are positioned as powders proximate the pre-sintered tungsten carbide/cobalt substrate prior to being sintered under ultra high pressure, high temperature (HP/HT) conditions.
One disadvantage of this prior art system is that the intermediate bonding layer, being in a fine powder form, presents a high surface area which promotes their rapid solution with the cobalt/tungsten eutectic causing severe crystal growth and eutectic pooling at the carbide/diamond or cBN interface.
Another disadvantage of the prior art is it is very difficult to create a very thin uniform layer of powdered material as an intermediate bonding layer. This causes non-uniform residual stresses at the carbide/diamond or CBN interface. This often causes delamination of the diamond or CBN layer.
The present invention circumvents the aforesaid problems by providing a thin solid layer of titanium carbide (TiC) bonded to the carbide substrate surface prior to HP/HT sintering. This thin continuous solid layer has a very low surface area compared to powdered TiC, therefore it does not readily go into solution with the cobalt eutectic. This results in a much lower level of grain growth and eutectic pooling, thereby practically eliminating diamond or CBN layer exfoliation.
The present invention also has an advantage over the prior art in that the lower and more uniform residual stresses provided by the solid thin and very uniform layer of TiC bonded to the carbide substrate materially reduces diamond layer delamination.